The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The generation of precise signals used for position location is the Key Performance Parameter (KPP) and main mission objective of the entire Global Positioning System (GPS). The GPS System involves GPS transponders that are typically carried by space vehicles (satellites), orbiting the Earth. A GPS receiver calculates its location and time by receiving location and time reports from four different space vehicles. The user location and time is determined by the intersection of four spheres, of which the centers of each of the spheres are the four space vehicle locations. The radii of the spheres are calculated by multiplying the speed of light (approximately 300,000 Km per second) by the time delays between space vehicles and the user receiver. This computation yields the user (i.e., GPS receiver) location, as well as its time.
User Range Error (URE) is the position error tolerance expressed in units of length (meters) experienced by the user when a position location is calculated by a GPS receiver. The coordinates of the determined location will typically be accurate to within a circle of URE radius.
A significant contributor to the URE is the signal delay variation as a function of temperature within the navigation payload aboard the GPS space vehicles. In this regard it will be appreciated that the space vehicles that carry the GPS transponders typically experience widely varying temperatures throughout the day. These temperature variations have a significant impact on the GPS transponder (or transponders) carried by a given space vehicle. More specifically, the electronic components that comprise the GPS transponder experience significant temperature variations throughout every twelve hour orbit. These temperature variations can introduce significant phase uncertainty in the navigation codes transmitted from a GPS transponder. The phase uncertainty will typically give rise to a time delay uncertainty that is directly proportional to the phase uncertainty. The time delay uncertainty can significantly affect the accuracy of a location determination made by a GPS receiver that is receiving the GPS codes and using them to determine its real time location.
Addressing the above-described signal delay errors caused by thermal variations in a GPS transponder output can be especially important in military applications. For example, it is known that every nanosecond of delay uncertainty between each space vehicle and the user translates roughly to a 30 cm (roughly a 12 inch) position calculation error. In a military application, for example, pin-point accuracy of a GPS guided munition may be important in striking a target with the munition.
Presently, attempts to minimize the temperature variation problem have focused on using heater elements to try and maintain the components of a GPS transponder at a constant temperature. This has necessitated the additional use of DC power, thus increasing the number of batteries and solar cells on the space vehicle that is carrying the GPS transponder. The additional batteries and/or solar cells add weight, cost and significant complexity to the GPS satellite. The added weight of the batteries and solar cells further adds to the cost of launching the space vehicle into orbit.